This invention relates to an apparatus for performing two and/or three-dimensional nuclear magnetic resonance (NMR) imaging, which imaging is based upon a function of the spin density distribution, the spin-lattice relaxation time (T.sub.1) and the spin-spin relaxation time (T.sub.2) of particular protons within a target. More specifically, the invention relates to a NMR imaging method and apparatus which compensates for inhomogeneities in the static magnetic field strength.
Atomic nuclei have net magnetic moments when placed in a static magnetic field, B.sub.0, at an NMR (Larmor) frequency .omega. given by the equation EQU .omega.=.gamma.B.sub.O (1)
in which .gamma. is the gyro-magnetic ratio, constant for each NMR isotope. The frequency at which the nuclei precess is primarily dependent on the strength of the magnetic field B.sub.0, and increases with increasing field strength.
Many different techniques are currently being investigated by which a characteristic image of a target, which might be part of a patient, can be effectively and efficiently obtained by nuclear magnetic resonance (NMR) imaging. Typically, the characteristic sought to be obtained is some function of the spin density distribution, the spin-lattice relaxation time (T.sub.1) and the spin-spin relaxation time (T.sub.2) of particular protons within the target. These protons are first excited by application of a magnetic field and a radio frequency (RF) pulse. Protons thus excited tend subsequently to relax, and during the process of relaxing generate a free induction delay (FID) signal. The above characteristic function of the relaxing protons within the target may be obtained by a Fourier transformation of this FID signal. By using the RF pulse being chosen to have a frequency spectrum corresponding to the Larmor frequency of the protons given by the formula .omega.=.gamma.B.sub.O, it is possible to excite protons in a single plane which may be a slice of the patient target.
Various NMR proton imaging techniques have been proposed, which involve four variable (three spatial variables, plus intensity). Such methods are based on two different principles. In one of these the data is gathered from a single localized region at a time so that many localized regions must be individually observed in order to obtain sufficient spatial data to construct an entire image.
More efficient data collection methods have been proposed in which the NMR imaging data is gathered simultaneously from many points, and the data received from the target contain spatial information about different parts of the target. An example of such a method is disclosed by Lauterbur in Nature, Vol. 242, Mar. 16, 1973, pages 190-191, in which NMR spectra which results from Fourier transforms of FID signals are derived from the target which is subjected to a magnetic field having a linear field gradient. Each individual spectrum represents a one-dimensional projection of the nuclear spin density in the target, integrated over planes perpendicular to the direction of the gradient. In order to obtain two-dimensional or three-dimensional images, spectra are derived for a series of different directions of the field gradient, and the results are subject to a process of "projection reconstruction".
There are other different imaging methods using a signal coding principle, for example, the Fourier imaging method described by Kumar, Welti and Ernst, "NMR Fourier Zeugmatography", Journal of Magnetic Resonance 18, (1975), pp. 69-83. In this reference, the spectral information is encoded by variable amplitude linear field gradients in respective orthogonal directions defining the slice of the target.
In general, NMR imaging apparatus requires a homogeneous field B.sub.0 which is stable for a long duration. However, in practice, the static magnetic field intensity may vary significantly in certain circumstance, especially due to drift in the static magnetic field power supply. These changes in magnetic field intensity may be both temporal and spatial and become serious with increasing lapse of time. As understood from the equation .omega.=.gamma.B.sub.O, the frequency of the NMR signal received from the target of course varies in accordance with any variation in the magnetic field intensity, and errors in the magnetic field result in geometrical distortions of the resulting image due to changes in the observed spectra.
Accordingly, it is an object of the invention to provide a method and apparatus for performing nuclear magnetic imaging which enables a reduction in aliasing of the spatial information in the NMR signals due to changes in the static magnetic field B.sub.0.
It is another object of the invention to provide a NMR imaging method and apparatus in which inhomogeneities in the magnetic field B.sub.0 are corrected by applying field offset gradients determined as a function of the decay time T.sub.2.sup.*.
It is still another object of the invention to provide a NMR imaging apparatus without a special stabilization system to maintain the static magnetic field uniform.